Barcode systems have been widely used in various aspects, such as automated logistic management, anti-counterfeit, labels, etc. In one application, a two-dimensional barcode or a QR code may be provided/printed on a poster or digital contents for protecting copyrighted content from illicit and unauthorized use. However, the additional printed barcode or QR code may adversely affect the overall esthetics of posters or digital contents.
Accordingly, it is a goal in the industry to embed information/data (e.g., anti-counterfeit data), into valuable printed matter (e.g., trademarks, pictures, etc.), such that the information/data is inconspicuously integrated in the printed matter without affecting the overall esthetics. Referring to FIG. 1, U.S. Pat. No. 8,594,453 discloses a method for bearing data in a halftone image in which a dot pattern of a cell is shown to consist of a plurality of pixels. By virtue of four shifted versions 502, 504, 506, 508 of the cell respectively representing four binary codes, such as 00, 01, 10 and 11, information may be embedded in the halftone image using binary codes, thereby preventing additional printing of barcodes or QR codes from affecting the overall esthetics. However, such an encoding method of moving the dots does not belong to traditional halftoning technique, and may have adverse effects on visual quality of the halftone image.
In addition, a conventional direct binary search (DBS) may be used to optimize visual quality of the halftone image. Referring to FIG. 2, the DBS iteratively/recursively performs on a central pixel 100 trials of swapping operations with eight neighboring pixels or toggling operation (black-white conversion, or vice versa), and obtains a minimum perceptual error using a human visual system based model that simulates human vision, thereby obtaining a halftone image that may achieve the highest visual similarity to the original grayscale image.
Another conventional halftoning technique is proposed in O. Bulan, G. Sharma, and V. Monga, “Orientation Modulation for Data Hiding in Clustered-Dot Halftone Prints,” IEEE Transactions on Image Processing, vol. 19, no. 8, pp. 2070-2084, August 2010 (hereinafter, Bulan reference). The Bulan reference performs screening with a modified Pellar threshold function to generate an original halftone image. The modified Pellar threshold function increases ellipticity of the halftone dot, thereby facilitating encoding by rotation of the halftone dot. However, referring to FIG. 5 of the Bulan reference, the dot orientation is not noticeably distinguishable in highlights, midtones (i.e., at gray-levels close to 50% area coverage) and shadows, and is unable to perform encoding by rotation of the halftone dot. For example, the halftone dots of the Bulan reference form a checkboard-like pattern, which are rotationally symmetric and unsuitable for encoding by rotation of the halftone dot.
FIGS. 8 and 9 respectively and exemplarily show a clustered-dot threshold matrix 81 and a dispersed-dot threshold matrix 91 which are two of threshold matrix that are the frequently used for screening, and their corresponding dot growth orders 82, 92. When a grayscale image is converted to a halftone image by the clustered-dot threshold matrix which is robust to printing artifacts that come from dot gain effects, the dots tend to cluster together. When a grayscale image is converted to a halftone image by the dispersed-dot threshold matrix, the dots tend to disperse.